In recent, as supply of mobile devices is widely spread, wireless LAN technology capable of providing mobile device users with wireless Internet services are being greatly spotlighted. The wireless LAN technology enables mobile devices, such as smart phones, smart pads, laptops, mobile multimedia players, and embedded devices, to be wirelessly connected to the Internet in a close distance.
Since initial wireless LAN technology supported a speed of 1˜2 Mbps by frequency hopping, spread spectrum, infrared ray communication and others using a frequency of 2.4 GHz through the Institute of Electrical and Electronics Engineers (IEEE) 802.11, the wireless LAN technology could have recently supported a speed of maximum 54 Mbps by applying orthogonal frequency division multiplex (OFDM). Besides, IEEE 802.11 is commercializing or developing standards for various technologies such as improvement of quality for service (QoS), access point (AP) protocol compatability, security enhancement, radio resource measurement, wireless access for a vehicular environment, fast roaming, mesh network, interworking with an external network, and wireless network management.
Of IEEE 802.11, IEEE 802.11b supports a communication speed of maximum 11 Mbps by using a frequency of a 2.4 GHz band. IEEE 802.11a, which has been commercially used after IEEE 802.11b, reduced influence of interference, compared to the frequency of the significantly complicated 2.4 GHz band, by using a frequency of a 5 GHz band, instead of the 2.4 GHz band, and improved the communication speed up to maximum 54 Mbps by using the OFDM technology. However, IEEE 802.11a has a drawback in that its communication distance is shorter than IEEE 802.11b. In addition, like IEEE 802.11b, IEEE 802.11g has been significantly spotlighted since it realizes the communication speed of maximum 54 Mbps by using the frequency of the 2.4 GHz band and meets backward compatibility. In terms of the communication distance, IEEE 802.11g is also superior to IEEE 802.11a.
In addition, IEEE 802.11n was established as a technology specification to overcome the limit of the communication speed that has been blamed as a weakness of the wireless LAN. The purpose of IEEE 802.11n is to increase a speed and reliability of a network and expand an operation distance of a radio network. More specifically, IEEE 802.11n supports a high throughput (HT) with a data processing speed of maximum 540 Mbps or more, and is based on the multiple inputs and multiple outputs (MIMO) technology using multiple antennas in both ends of each of transmission and reception units in order to minimize transmission errors and optimize a data speed. In addition, this specification may use a coding method that transmits several overlapping copies in order to improve data reliability, and furthermore, orthogonal frequency division multiplex (OFDM) in order to increase a speed.
As supply of the wireless LAN increases, and applications using the wireless LAN are diversified, there has been recently increasing necessity for a new wireless LAN system to support a higher throughput (very high throughput; VHT) than the data processing speed supported by IEEE 802.11n. Especially, IEEE 802.11ac supports a broad bandwidth (80 MHz to 160 MHz) in the 5 GHz frequency. The IEEE 802.11ac standard is defined only for the 5 GHz band, but initial 11ac chipsets would also support the operation in the 2.4 GHz band for lower compatibility with existing 2.4 GHz-band products. In this case, 802.11ac supports a bandwidth of from 2.4 GHz to maximum 40 MHz. Theoretically, according to this standard, a wireless LAN speed of multiple devices can be at least 1 Gbps, and a maximum single link speed can be at least 500 Mbps. This is realized by expanding radio interface concepts accepted in 802.11n such as broader radio frequency bandwidth (maximum 160 MHz), more MIMO spatial streams (maximum 8 streams), multiple user MIMO, and high-density modification (maximum 256 QAM). In addition, there is IEEE 802.11ad, which transmits data by using a 60 GHz band, instead of existing 2.5 GHz/5 GHz. IEEE 802.11ad is a transmission specification for providing a speed of maximum 7 Gbps by using a beamforming technology, and suitable for high bit-rate video streaming such as a large amount of data or uncompressed HD videos. However, the 60 GHz frequency band is disadvantageous in that it cannot easily pass through obstacles, and thus, should be used only among devices in a short distance space.
Meanwhile, Korean Patent No. 0643766 (Title of Invention: High-Speed Handover Method Optimized for IEEE 802.11 Network) describes classifying APs based on signal strength of peripheral APs, and determining an AP to implement handover on this basis.